High speed collection, digitization and storage of analog signals, such as waveforms, in digital form has already eclipsed the speed of presently available analog-to-digital converters (ADC's). To achieve higher performance in a data acquisition circuit, one or more digitizing channels may be added in parallel so that successive data points of the sampled signal may be converted into digital form by different channels using different ADC's. The technique of "time-shifting" the added digitizing channels allows each channel to operate on data points from different moments in time, thus reducing the sampling rate of each channel and thereby reducing the bandwidth requirement on each individual ADC. In principle, it enables extremely fast analog-to-digital conversion, assuming, among other things, that enough parallel channels are available to achieve the desired speed. The technique is similar to that of interleaving the memory units in a high-performance computer.
However, there are problems associated with this type of data acquisition circuit. One of the more significant problems is that of correlating the resulting digitally encoded data. Unless the parallel digitizing channels have the same signal parameters, the data points encoded by different channels will have different gains and different zero frequency (dc) offsets. Of course, it is not generally possible to have identical circuits, because there will always be some manufacturing tolerances which permit differences between the components used in the various channels, so some form of compensation is needed to remove the unwanted parameter variation.
One way to compensate is to ignore the discrepancies in the data until all of the data points are collected and then perform some type of post-calibration, such as by reading back all of the data and correcting each data point for the particular gain and dc offset it received This may work in some cases, but it greatly complicates the process of correlating the data values from the various digitizing channels and reduces both the effectiveness and efficiency of the data collection. It could also result in some data values being lost if the input signal goes out of range of one of the digitizing channels. This effectively limits the useful dynamic range of the system to those signals which overlap all of the parallel channels. Unfortunately, as more parallel channels are added to achieve higher sampling speeds, it becomes more likely that some signals will not overlap all of the channels and that a reduction in dynamic range will occur.
Another way to compensate for discrepancies caused by differences between the digitizing channels is to perform manual calibration of the individual channels prior to the collection of data. Manual calibration of the circuit will work, but only if all of the operating parameters of all of the channels remain constant between calibration times. Unfortunately, the operating parameters may change as a result of changes in temperature, sampling rate, input sensitivity, etc. Since these changes are almost inevitable in a general purpose instrument, the assumption of constant parameters will probably be invalid. As a consequence, manual calibration offers little assurance of valid data. Manual calibration also requires the time and talents of someone to perform the calibration, which adds to maintenance costs and downtime.
A third way to compensate the channels is to provide some form of automatic calibration. One technique for doing this incorporates a wideband gain-programmable amplifier whose parameters are controlled by a feedback circuit. Such feedback circuits are generally straightforward to design but the programmable amplifiers are complex, costly devices which may be unsuitable for many applications. In particular, the cost and complexity may become prohibitive if many time-shifted digitizing channels are used, because each channel will require its own programmable amplifier.